Hybrid electric power systems have been developed to combine an internal combustion engine power system with an electric power system typically including a battery and an electric motor. Hybrid electric power systems have been applied to vehicles (known as hybrid electric vehicles or HEVs) to provide benefits such as improved fuel efficiency and lower exhaust emissions in comparison with conventional vehicles powered by internal combustion engine power sources alone.
Existing HEV systems vary in their configuration and the manner in which the internal combustion engine and electric power subsystems are combined to power the HEV. Existing HEV system configurations may be generally described as either parallel or series hybrid electric systems. In a parallel HEV system configuration, generally the internal combustion engine and electric power subsystems are combined so that the internal combustion engine is a primary source of torque to drive the wheels of the HEV.
Commonly, in a parallel hybrid electric system configuration, the internal combustion engine is a primary source of drive wheel torque, typically through a conventional or modified mechanical transmission means, whereas the electric power subsystem may be either a co-primary or a secondary source of torque to the drive wheels of the HEV. An example of a conventional parallel hybrid electric system configuration is disclosed in U.S. Pat. No. 5,789,877, the contents of which are herein incorporated by reference.
Alternatively, in a series HEV system configuration, generally the electric power subsystem and particularly the electric motor are the primary source of torque to drive the wheels of the HEV. In a series hybrid electric configuration, the internal combustion engine is typically used principally or solely to drive an electrical generator that supplies electrical energy to the battery and electric motor of the electrical power subsystem, and the electric motor is the primary or in some cases only power source to supply torque directly to the drive wheels of the HEV. Examples of conventional series hybrid electric system configuration is disclosed in U.S. Pat. No. 5,589,743, and in PCT patent publication No. WO 2006/093515, the contents of which are herein incorporated by reference.
HEVs employing a series hybrid electric power system configuration may provide certain advantages over parallel configurations for some types of HEVs. For example, an advantage of a series HEV configuration is that the internal combustion engine may be located anywhere in the vehicle because it is not necessary to align the engine with a mechanical drive train connected to the drive wheels of the HEV, since the internal combustion engine typically does not directly provide torque to the drive wheels. Another potential advantage of a series HEV configuration is that the internal combustion engine may be optimally configured to run at a relatively constant speed while driving an electrical generator, such that the efficiency of the engine is optimized, rather than requiring the engine to run at widely varying speeds to mechanically drive the wheels of the HEV. Particularly with diesel internal combustion engines, the fuel efficiency of the engine when optimized to run at a constant speed may be significantly increased relative to a similar engine run over a wide range of speeds corresponding to the driving cycle of a vehicle.
However, a potential disadvantage of series HEV system configurations is that the reliability of the entire drive train system for driving the wheels of the HEV is primarily or solely dependent on the reliability of the electric power subsystem, typically including a generator powered by the internal combustion engine, an energy storage system or battery, and a drive motor supplying torque to the drive wheels of the HEV. If the electric drive motor and/or battery fail, there is typically no source of torque to drive the wheels of the HEV, and if the electric generator fails, the drive motor can only drive the wheels of the HEV for as long as the electrical energy stored in the battery or other energy storage system is available. Additionally, in some parallel HEV system configurations including an electric motor which is a co-primary source of drive wheel torque, the reliability of the HEV system to drive the wheels of the HEV is at least partially reliant on the reliability of the electrical power subsystem, including the generator and electric drive motor. Therefore, the improvement and optimization of HEV drive systems, particularly series HEV systems, is dependent on the reliability of the electrical motor and generator components, indicating a need for HEV electrical systems with improved reliability.
Further, particularly for HEVs designed for use in harsh or dangerous environments, where reliability of the HEV is critical, and where potential for damage to the HEV electrical system components exists, HEV electrical systems with improved reliability, durability and survivability are needed. For example, for HEV systems designed for military or other conflict-related use where reliability is critical, the potential for damage to the HEV electrical power system components from wear and/or weapon-related causes indicates a need for HEV electrical systems with improved survivability, fault tolerance, and graceful degradation operating characteristics.